1. Field of the Invention
The present invention relates to an optical disk apparatus that can optimize frequency of high frequency current to be superimposed so as to realize good reproduction performance within a wide irradiation power range necessary for reproducing a single-layer disk and a multi-layer disk.
2. Description of the Related Art
As a method for reducing noise of the semiconductor laser in a reproduction process, a high frequency superimposing method is known. In this method, high frequency alternating current (high frequency current) output from a high frequency superimposing circuit is superimposed on direct current output from a semiconductor laser device drive circuit, thereby being supplied to the semiconductor laser for pulse oscillation. Thus, interference between a light flux emitted from the semiconductor laser and a light flux returning to the semiconductor laser via an optical system of the optical disk apparatus is reduced so that occurrence of the laser noise can be suppressed.
Here, Japanese Patent Application Laid-Open No. 2007-134003 discloses a technology in which an information recording surface of an optical disk is divided into multiple concentric regions, and the irradiation power is changed in accordance with each of the regions in the reproduction process to thereby switch, in accordance with the changed irradiation power, output amplitude or frequency of the high frequency current.
However, the above-mentioned conventional technology does not specifically disclose optimizing of the frequency of the high frequency current to be superimposed in accordance with the irradiation power of the semiconductor laser.
Here, the inventor of the present invention found that the following problems arise if the high frequency superimposing method is to be performed with a wide range of the irradiation power that is necessary for reproducing a single-layer disk and a multi-layer disk compared with the case where a predetermined recording layer is reproduced as in the above-mentioned conventional technology.
For instance, it is supposed that a frequency of high frequency current optimized in a high irradiation power range that is necessary for a multi-layer disk is superimposed in a low irradiation power range that is necessary for a single-layer disk. In this case, intrinsic noise of the semiconductor laser increases in the low irradiation power range so that the reproduction performance is deteriorated.
On the contrary, it is supposed that a frequency of high frequency current optimized in a low irradiation power range that is necessary for a single-layer disk is superimposed in a high irradiation power range that is necessary for a multi-layer disk. In this case, influence of the return light increases in the high irradiation power range so that the reproduction performance is deteriorated.
As described above, it is difficult to satisfy good reproduction performance in both ranges including the high irradiation power range necessary for the multi-layer disk and the low irradiation power range necessary for the single-layer disk.